Hydraulic fracturing fluids have been used widely in the stimulation of oil and gas wells. Proppant transport and retained conductivity are important qualities of fracturing fluids. The fluids are typically viscosified with a natural or synthetic polymer to improve the ability of the fluids to transport proppant materials. Water or hydrocarbons have been commonly used as base fluids for fracturing. While usually effective, water-based fluids can be harmful to certain types of formations, and are not effective at removing excess water from a well (removing “water blocks”).
It is preferable that a fracturing fluid be compatible with carbon dioxide or other gasses. As used herein, the fluid or the polymer therein is “compatible” if it does not form a precipitate upon contact with the gas. Addition of carbon dioxide to a fracturing fluid provides gas pressure to assist in returning fluids to the wellbore after treatment.
The use of alcohols as base fluids has been previously suggested. Advantages of alcohols over water-based fluids include low freezing points, low surface tensions, high water solubilities, high vapor pressures, and good compatibility with formations. Alcohols have several potential safety issues relating to their low flash points, high vapor densities, and invisibility of flame. These safety issues can be properly addressed by skilled operators to minimize any associated risks.
Methanol foams have been prepared using synthetic polymers (polyacrylamide and polyethylene oxide). Attempts were made to crosslink the gelled methanol using metal crosslinkers. These include the use of titanium crosslinked fluids marketed by several service companies (for example, Methofrac 3 by Fracmaster, Ltd. (acquired by BJ Service Company) and Methofrac XL by BJ Services Company). These typically contained several percent of water, either for gelling and/or for breaking the gels. The titanium crosslinked polymers in the fluids did not break completely without the water and also did not perform well at temperatures greater than 90° C. Without water, this polymer system was not compatible with carbon dioxide.
A modified guar polymer was reported to dissolve in anhydrous methanol and crosslinked with a borate complexor. The complex was broken with an oxidizing breaker. This polymer as well as the borate crosslinker were not compatible with carbon dioxide (i.e. formed a precipitate and the borate crosslink was reversed).
SPE 13565 (S. C. Crema and R. R. Alm, 1985; presented at the International Symposium on Oilfield and Geothermal Chemistry, Phoenix, Ariz., Apr. 9–11, 1985) describes the preparation of foamed anhydrous methanol. The foamed material is offered for the stimulation of water sensitive formations. The foams contain a fluorosurfactant and a foam extender. The foam extender allows a reduction in the amount of fluorosurfactant needed. Example foam extenders include oxyalkylated fatty alcohols and amines or polyethers containing ethylene and propylene oxide units. Foamed fluids have limited viscosity, and as a result, their practical application is limited.
SPE 14656 (C. M. Fairless and W. Joseph, 1986; prepared for presentation at the East Texas Regional Meeting of the Society of Petroleum Engineers, Tyler, Tex., Apr. 21–22, 1986) describes the use of a two-phase structured system for the treatment of wells. Vaporized carbon dioxide is dispersed as an internal phase in a gelled complexed methanol external phase to produce a foam. The foams were used to treat water sensitive formations.
SPE 22800 (J. E. Thompson et al., July 1992) suggests a continuous mix process for gelling anhydrous methanol. The continuous mix process is suggested as a less risky alternative to batch processing. Additionally, the continuous process achieved full fluid viscosity in a reduced amount of time, and the performance of the produced materials was similar.
SPE 27007 (J. M Hernandez et al., 1994; prepared for presentation at the Latin American/Caribbean Petroleum Engineering Conference, Buenos Aires, Argentina, Apr. 27–29, 1994) presents a comparison of methanol and other fluids as fracture fluids in gas wells. Methanol was shown to provide additional stimulation near the fracture faces, decrease the saturation of water in the zone, and increased the gas permeability of the formation
SPE 35577 (D. B. Bennion, et al., 1996; prepared for presentation at the Gas Technology Conference, Calgary, Alberta, Canada, Apr. 29–May 1, 1996) offers a review of efforts taken to obtain natural gas in low permeability sandstone and carbonate formations. Methanol is suggested as being able to significantly reduce interfacial tension between water-gas or oil-gas systems.
SPE 70009 (Mark R. Malone, 2001; prepared for presentation at the SPE Permian Basin Oil and Gas Recovery Conference, Midland, Tex., May 15–16, 2001) describes the use of crosslinked methanol fracturing fluids in water-sensitive formations. A crosslinked methanol system was prepared using hydroxypropyl guar, encapsulated ammonium persulfate breaker, and liquid carbon dioxide. Case histories were described using the fracturing fluids in test wells.
Despite the reported art, there exists a need for non-aqueous fracturing fluids that are compatible with carbon dioxide.